Conventionally, inorganic oxide powder has been employed as, for example, a filler for imparting to an object characteristics such as reinforcement, flame retardancy, high thermal conductivity, and insulating property, a filler for a semiconductor sealing material, a viscosity adjusting agent, a fluidity improving agent, or a charge regulating agent; or has been employed in, for example, a coating material composition or a cosmetic composition. Particularly, spherical inorganic oxide powder has been suitably employed as, for example, a filler of high thermal conductivity or a filler for a semiconductor sealing material, because the oxide powder exhibits excellent fillability and fluidity and is easy to handle. For example, spherical alumina is employed as a thermally conductive filler, by virtue of its high thermal conductivity, and spherical silica is employed as a filler for a semiconductor sealing material, by virtue of its high purity.
In a technique for producing spherical particles, an inorganic oxide serving as a raw material is brought into a high-temperature flame to thereby melt the oxide, and the thus-melted oxide is formed into spherical particles by means of surface tension of the melted oxide. Instead of an inorganic oxide, a metal may be employed as a raw material. In such a case, high-temperature oxidation and melting-spheroidization of a metal occur concurrently.
In connection with the aforementioned technique, among other methods, the following methods have been proposed: a method in which alumina powder and silica powder are simultaneously sprayed into a flame, so as to allow reaction between a sodium component of the alumina powder and the silica powder to proceed, thereby producing spherical alumina of low sodium content (Japanese Patent Application Laid-Open (kokai) No. 2001-199719); a method in which alloy powder is combusted in an oxygen-containing gas atmosphere, to thereby produce a spherical composite oxide (Japanese Patent Application Laid-Open (kokai) No. 63-185803); and a method in which a slurry of aluminum hydroxide powder or aluminum oxide powder is sprayed into a flame, and the resultant fine powder is collected at a temperature of 500° C. or higher (Japanese Patent Application Laid-Open (kokai) No. 2001-19425).
A surface treatment will next be described.
In general, the surfaces of inorganic particles exhibit hydrophilicity. In the case where inorganic particles serving as a filler are added to an organic polymer material, hydrophobicity must be imparted to the surfaces of the inorganic particles. When hydrophobicity is not imparted to the surface of the filler, sufficient adhesion fails to be established at the interface between the filler and the organic polymer material, and the filling ratio of the filler is lowered. In order to impart hydrophobicity to the surfaces of the inorganic particles, the particles are subjected to surface treatment by use of, for example, a silane coupling agent or a titanium coupling agent. Such a surface treatment agent has a molecular structure including a hydrophobic group such as an alkyl group, and a hydrolyzable functional group such as a methoxy group or an ethoxy group. When a methoxy group or an ethoxy group of such a surface treatment agent undergoes hydrolysis, for example, a silanol group is formed, and the thus-formed silanol group is bonded to the surfaces of the inorganic particles. Since the thus-bonded molecules of the surface treatment agent have a hydrophobic group such as an alkyl group, the surfaces of the inorganic particles exhibit high hydrophobicity. In general, the above-described surface treatment technique is widely employed (e.g., NUC Silane Coupling Agent Catalogue produced by Nippon Unicar Co., Ltd.).
Composite particles will next be described. In many cases, composite particles are formed through bonding between large-size particle (hereinafter referred to as “mother particles”) and small-size particles (hereinafter referred to as “child particles”). Mother particles are employed for effectively obtaining functions of child particles. In the case where no great difference in particle size is observed between particles for forming composite particles, fine particles having functions of interest are called “child particles,” and particles employed for effectively obtaining the functions are called “mother particles.”
Mother particles are bonded to child particles by means of, for example, a high-speed gas impact technique for mechanically bonding mother particles to child particles (Japanese Patent Publication (kokoku) No. 3-2009 and Japanese Patent Application Laid-Open (kokai) No. 6-210152), or a mechanofusion technique (Japanese Patent No. 2672671). Specifically, such bonding is attained by means of, for example, a Hybridization System (registered trademark, product of Nara Machinery Co., Ltd.) or a Mechanofusion System (registered trademark, product of Hosokawa Micron Corporation). Examples of the apparatus employed for attaining such bonding include a rolling ball mill, a high-speed rolling mill, and a medium stirring mill. In any of these apparatuses, the surfaces of mother particles and child particles are activated by means of, for example, impact energy, friction energy, or shear energy of milling media, which energy is applied to the mother particles and child particles, whereby the mother particles are bonded to the child particles. During the course of such treatment, the mother particles may be milled and formed into fine particles.
An object of the present invention is to provide a novel method for modifying the surfaces of particles, which method enables production, at low cost, of a filler suitable for use in a highly viscous composition such as an organic polymer composition or a silicon-containing polymer composition. Conventionally, inorganic powder has been employed as a filler in a composition, in order to enhance characteristics of the composition, such as thermal conductivity, flame retardancy, and insulating property. In order to enhance such characteristics of the composition as much as possible, the inorganic powder (filler) is required to exhibit high fillability. Meanwhile, the inorganic powder must exhibit high fluidity, from the viewpoint of handling of the powder during the course of production of the composition. In view of the foregoing, a spherical filler of high fillability has preferentially been employed in a matrix, and adhesion of the filler to the matrix has been enhanced by subjecting the filler to surface treatment.
However, in a conventional technique, a process for producing spherical powder of high fillability is separated from a process for surface treatment of the powder, leading to very low productivity and high production cost. The reasons for such separation of the processes are as follows. Although production of spherical powder requires high-temperature thermal treatment, a conventional surface treatment agent undergoes decomposition at high temperature, and thus difficulty is encountered in carrying out production of the powder and surface treatment thereof in a single process.